for e.g. anime where the characters mostly look the same = bad, imo. animes where the characters look unique from one another, the background has a lot of detail = good.

That seems more about art style than animation.. The two are separate although related. Its possible to have good art style and poor animation, or conversely weak art style and good animation.

For instance Kids on the Slope had rather plain art, although I'm not going to call it bad at all, but the animation in that show is fantastic. Many long running shounen get put in weird situation where the art style is actually consistent, but runs the problem of the animation spiking and lowering in quality throughout the series. I mean look at Rock Lee vs Gaara in the Chuunin exams vs some of the other non filler combat in Naruto. The art style didn't change at all but the animation quality fluctuates wildly. On the same note in those type of shows I can often spot studio filler right off the bat as I can usually spot art style inconsistencies quickly(among other things).

Not really sure if the OP, or you for that matter, really cares about the difference at all. I just figured I'd put my 2 cents in since the terms are often conflated.

1) Consistency and uniqueness of character models. I've seen shows where I've had to do a double-take, because for a second one of the characters looked completely different than normal. Uniqueness also matters: do the characters look like every other character from every other anime... or even worse, are they indistinguishable from other characters in the same anime?

Most shows that I think of as having bad animation fall into one of those two traps.

2) Does the animation match the story and dialogue? This may sound obvious, but I've seen shows get this wrong before. Pretty characters look ugly, big breasted characters are thin, skinny characters have indistinguishable body types from everyone else, etc.

This is the other trap that poorly animated shows fall in to.

3) Do the artistic choices add anything to the story? Shaft does this very well. Bakemonogatari uses the lighting, the backgrounds, and the settings to either increase the either the playfulness or the eeriness of a particular scene. In a non-shaft example, Usagi Drop looks like a watercolor painting, which reinforces the sense of childishness of the series as a whole.

It's not all that common to see this done exceptionally well, but this is often the difference between "good" animation and "great" animation in my mind.